Protective circuit for a direct-coupled amplifier

Abstract

A low pass filter is connected to a junction of a direct-coupled amplifier. The amplifier has a single-ended push-pull output stage consisting of two output transistors and series-connected two resistors. The output transistors are connected to a positive and negative d-c power sources, respectively. The series-connected two resistors are interposed between the two output transistors and defines an intermediate point therebetween at which substantially zero d-c voltage prevails when the amplifier is functioning normally. The junction of the amplifier, to which the low pass filter is connected, is between one of the output transistors and the series-connected resistors. The low pass filter is supplied with the voltage at the junction and applies its output to following isolating means. The means responds to the output of the low pass filter for isolating the intermediate point from a load device connected thereto.

What is claimed is: 
     
       1. A protective circuit for a direct-coupled amplifier and for a load device connected to said direct-coupled amplifier, said direct-coupled amplifier comprising: a positive and a negative power sources;   two output transistors connected to said positive and negative power sources, respectively;   series-connected two resistors defining an intermediate point therebetween at which substantially zero voltage prevails when said direct-coupled amplifier is functioning normally, and being interposed between said two output transistors; and   an output terminal interconnecting said intermediate point and said load device;   said protective circuit comprising:   a low pass filter connected to the junction between one of said output transistors and said series connected two resistors; and   means connected to said low pass filter for isolating said intermediate point from said output terminal in response to the output of said low pass filter.   
     
     
       2. A protective circuit as claimed in claim 1, wherein said means comprises an absolute value amplifier connected to said low pass filter, a driver connected to said absolute value amplifier and an isolating device connected to and driven by said driver in response to the output of said driver for isolating said intermediate point from said output terminal. 
     
     
       3. A protective circuit as claimed in claim 2, wherein said isolating device is a relay the contacts of which are provided between said intermediate point and said output terminal. 
     
     
       4. A protective circuit for a direct-coupled amplifier and for a load device connected to said direct-coupled amplifier, said direct-coupled amplifier comprising: a positive and a negative power sources;   two output transistors connected to said positive and negative power sources, respectively;   series-connected two resistors defining an intermediate point therebetween at which substantially zero voltage prevails when said direct-coupled amplifier is functioning normally, and being interposed between said two output transistors; and   an output terminal interconnecting said intermediate point and said load device;   said protective circuit comprising:   a first low pass filter connected to the junction between one of said output transistors and said series-connected two resistors;   a second low pass filter connected to the junction between the other of said output transistors and said series-connected two resistors;   a differential amplifier having two inputs connected to the output of said first and second low pass filters, respectively; and   means connected to the output of said differential amplifier for isolating said intermediate point from said output terminal in response to the output of said differential amplifier.   
     
     
       5. A protective circuit as claimed in claim 4, wherein said means comprises an absolute value amplifier connected to said differential amplifier, a driver connected to said absolute amplifier, and an isolating device connected to and driven by said driver in response to the output of said driver for isolating said intermediate point from said output terminal. 
     
     
       6. A protective circuit as claimed in claim 5, wherein said isolating device is a relay the contacts of which are provided between said intermediate point and said output terminal.